1. Field of the Invention
This invention relates generally to seismic cable systems, and, more particularly, to a robust and reliable seabed seismic cable system.
2. Description of the Related Art
Subsurface hydrocarbon accumulations are increasingly found in geologically complex areas. The ability to conduct accurate seismic surveys may help improve the discovery rates and even the production of such accumulations. Seismic surveying is a method of stimulating a geological subsurface formation with, e.g., electrical, magnetic, and/or acoustic signals to acquire seismic data about the formation. From this data, one can predict whether the formation contains hydrocarbon deposits and, if so, where.
One type of seismic survey is generally referred to as a “marine” survey because it is typically conducted at sea, although this is not necessarily always the case. During marine seismic surveys, seabed seismic cable systems are deployed to the bed of a sea, lake, river, or marsh. The water depth may range from several thousand meters up to the water/land transition zone. Water currents of considerable speed may pass over the cable system and create instability and deterioration of the seismic data quality.
Seabed seismic cable systems generally are designed to meet two conflicting goals. First, the cable system must be robust and resistant to damage. For example, the cable system must survive and operate at great water depth. Also, the cable system may be roughly handled during deployment and retrieval. Second, the cable system should be sensitive to acoustic vibrations and not compromise the quality of data recorded by the sensor units. To design and construct a robust but sensitive cable requires balancing robustness and sensitivity through a large number of tradeoffs.
Although there are several types of seabed seismic cables, there are generalities in construction. A seabed seismic cable includes three main elements: stress members, leads, and a sheath One or several stress members take the tension that can be applied to the seabed seismic cable during deployment and retrieval operations to protect other elements of the seismic cable. The leads, which may be electrical or optical, transmit power and/or data, in analogue or digital format, along the cable for collection and processing, e.g., on a survey vessel. The sheath is a skin, jacket or extrusion matrix protecting the seabed seismic cable against, notably, water ingress.
A seismic cable will generally have sensor modules distributed along its length. Inside the sensor module are sensors. Examples of sensors are geophones, accelerometers, hydrophones, tilt meters, magnetometers. The sensors can include electronics conditioning the signal and/or digitizing it. The sensors can be connected by the leads mentioned above, transmitting the sensor data through the seismic cable to electronics modules located along the cable or between cable sections, or can be connected by a data bus.
One type of seabed cable is known as an “ocean bottom cable” (“OBC”), and is typically equipped with “takeouts.” For example, the use of a take out anchor and protective cover is described in U.S. Pat. No. 6,294,727 to Orlean. A full length of conventional OBC is seismic built, the jacket is then opened at the location were the sensors are located, and leads are extracted from the cable to form a take-out and connected to the sensors. The sensors are then attached to the cable.
These types of cable are prone to water intrusion, electrical leakage, and wire kinking, as the take-outs are submitted to a high level of strain during cable handling. These cables usually have an asymmetric cross-section at the sensors, and the response will change depending on how the sensors rest on the seafloor. These types of cables also expose the seismic receivers and the takeouts to a number of potentially damaging obstacles on the seabed, thereby reducing the reliability of the collected data. Furthermore, because the takeouts are extracted from the cable and not a separate component, the entire cable may need to be replaced if the takeouts are damaged, which can be expensive and time-consuming.
Another type of seabed seismic cable system is commonly referred to as a logging type cable. Logging type cables typically have a full electrical/optical termination at each sensor unit, resulting in a high number of connection points. The high number of connection points negatively impacts the cable's reliability. Furthermore, the increased number of terminations makes the sensor unit large and heavy, which negatively impacts data quality.
Yet another type of seabed seismic cable system comprises conventional cables with sensor units integrated inside a protective cable jacket One variation is known as a “streamer type” cable. The streamer type cable is an evolution of a towed seismic streamer for deployment on the seabed. The streamer type cable comprises spacers, sensor units, and a filler, which usually is oil. The streamer type cable has a constant diameter and therefore occupies a large volume when stored. An alternative variation is known as a “solid cable.” Examples of the solid cable include a constant diameter solid cable disclosed in U.S. Pat. No. 6,041,282 to Wardeberg et al. and a variable diameter solid cable disclosed in U.S. Pat. Nos. 6,333,897 and 6,333,898 to Knudsen et al. Both the constant diameter and the variable diameter solid cables pose potentially serious drawbacks. For instance, the constant diameter solid cable is extremely large, heavy and stiff, while the variable diameter solid cable is difficult to manufacture.
In U.S. Pat. No. 5,265,066 to Svenning et al., a seismic seabed cable including at least one pulling cable, a data cable, and a sleeve which surrounds the data cable and the pulling cable is disclosed. Groups of geophones and position metering instruments are placed along the cable with the position metering instruments and geophones being disposed in a geophone sphere. However, the volume and size of the seismic seabed cable described by Svenning may compromise data quality and robustness of the cable while the cable is being handled.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.